The discovery of monolayer graphene sheets proved that two-dimensional materials with only one layer of atoms can stably exist with or without substrate support. Mono- and few-layered graphene sheets have since stimulated much excitement not only because they are novel materials of excellent mechanical, electrical, and thermal properties, but also because their preparation could be readily achieved in large scale using known or intuitively derived methods. For example, few-layered graphene sheets can be prepared by treatment of graphite using strong oxidative acid for carbon oxidation and thus exfoliation to form few-layered graphite oxide, which could then be reduced to graphene.
Hexagonal boron nitride (h-BN), sometimes called white graphite, is structurally analogous to graphite, with the layered sheets similarly held together by van der Waals forces. Compared to the all-carbon structure of graphene, each hexagonal boron nitride (h-BN) sheet is composed of boron and nitrogen atoms alternatively positioned in the planar hexagonal crystal structure. The interlayer structure of h-BN is such that the boron and nitrogen atoms in adjacent layers eclipse one another due to the polarity of the two atoms, forming so-called AB stacking. Different from graphite, h-BN is highly inert to oxidative environment. More specifically, it is stable against heating in oxygen environment up to 800° C. or above and also strong oxidative acid treatment.
Similar to the exfoliated graphene nanosheets structures, the mono- or few-layered h-BN may provide unique mechanical, thermal, and electrical properties that are superior to that of the bulk h-BN material. Exfoliated h-BN nanosheets have excellent thermal properties, but are electrically insulating, different from their conductive graphene counterparts.
Prior art methods of forming thin h-BN sheets include chemical vapor deposition of h-BN on a metal surface, micromechanical cleavage, and sonication. With chemical vapor deposition, the resultant two-dimensional h-BN nanostructures are supported on the metal surfaces and thus not available in freestanding forms for applications such as for coatings and composites. Micromechanical cleavage from h-BN powder or single-crystal was achieved, in one example, by applying adhesive tape to h-BN powder and then attaching the tape to a 300 nm thick silicon oxide substrate. Only minute amounts of few-layered h-BN nanosheets were obtained on the substrate. No mono-layer h-BN was obtained. Sonication of h-BN single-crystal in 1,2-dichloroethane in the presence of a conjugate polymer [poly(m-phenylenevinylene-co-2,5,-dictoxy-p-phenylenevinylene] yielded h-BN with few layers. No free-standing mono-layer h-BN was found. Sonication of h-BN powder in a polar organic solvent, i.e. N,N-dimethylformamide (DMF), without surfactant yielded h-BN nanosheets with few layers (as few as 3 layers).